1. Field of the Invention
The present invention relates generally to solar energy collecting apparatus and specifically relates to the type of such collecting apparatus which concentrates solar energy on a focal region. Further, the solar energy collecting apparatus of the invention is preferably of the fixed type that does not change attitute with daily or seasonal positional changes of the sun.
Since reserves of fossil fuel resources are being depleted at increasing rates, greater emphasis is now being placed on effectively and efficiently collecting and using energy radiated from the sun. Solar energy is, for practical purposes, limitless. However, collecting solar energy in sufficient quantities to supply current needs has proven difficult.
Solar energy collecting systems are now most practically and economically used for water heating purposes but may be adapted to supply energy for other purposes such as space heating and air conditioning. Solar energy collecting apparatus used in such applications now usually fall into one of two basic categories. One such category comprises flat-plate solar energy collecting apparatus that employ an absorber having a flat surface provided with a black or selective coating that is capable of receiving clear or diffuse solar energy from a large range of angles without any provision for movement of the surface. The second category comprises concentrating solar energy collecting apparatus that employ reflective elements to focus solar energy on a focal region. Generally the temperature or grade of heat that may be derived from the solar energy collected with a concentrating collector is much greater than that achieved with flat-plate collectors.
2. Description of the Prior Art
Many forms of concentrating solar energy collecting apparatus have been proposed. Typically, such apparatus employ a reflector, which is often a parabolic or circular cylindrical mirror, that focuses solar energy on an absorber, which usually is a tube or assembly of tubes through which a heat conducting fluid may be conducted. Some concentrating solar energy collecting apparatus are designed to be fixedly mounted yet nevertheless reflect large amounts of solar energy to the absorber during a clear solar day. Examples of this type of apparatus are disclosed in U.S. Pat. Nos. 4,011,855 (Eshelman), 4,059,094 (Barrio de Mendoza), 4,024,852 (L'Esperance et al.), and 4,003,366 (Lightfoot).
Other concentrating collecting apparatus are designed to move in coordination with movement of the sun. This type of apparatus is said to "track" the sun and examples of it are disclosed in U.S. Pat. Nos. 4,026,273 (Parker), 4,058,110 (Holt), 4,011,858 (Hurkett), 4,038,972 (Orrison), 4,067,319 (Wasserman) and 4,077,392 (Garner).
Still other concentrating solar energy collectors are disclosed in "Solar Concentrating Collectors", Proceedings of the ERDA Conference on Concentrating Solar Collectors, Georgia Institute of Technology, Atlanta, Ga., Sept. 26-28, 1977. One such collector, disclosed in Cole, Allen, Levitz, McIntire, and Schultz, Performance and Testing of a Stationary Concentrating Collector delivered during the Proceedings (see pages 3-31 through 3-37) is called the Compound Parabolic Concentrator (CPC) because it employs a reflector having adjacent parabolic portions, adjacent side walls of which intersect at a peak or cusp. An absorber is mounted directly on the cusp. Problems with CPC collectors are discussed in Patton, Design Considerations for a Stationary Concentrating Collector (see pages 3-37 through 3-44) also delivered during the Proceeding.
Concentrating collectors are said to be either imaging or non-imaging, and both types may be oriented either in the north-south or east-west direction. Imaging collectors are only efficient when positioned nearly normally to incoming insolation while non-imaging collectors have a finite acceptance angle and insolation within that angle, including diffuse radiation, may be accepted. Imaging concentrating collectors must move to track the sun when installed in either orientation. Non-imaging concentrating collectors may be designed to track the sun but can be designed to be fixed when installed in the east-west orientation.
Generally, imaging concentrating collectors do not work effectively in diffuse light since much of the reflected energy necessarily misses the absorber target. Non-imaging concentrating collectors more efficiently collect energy from diffuse light than do imaging concentrating collectors by providing a larger absorber target for reflected energy, but are not as efficient in this regard as are flat-plate collectors.
Tracking collectors are capable of receiving solar energy from a large range of angles since they can "follow the sun". However, because they must be spaced so one collector unit does not adversely shade an adjacent unit, the ratio of net collector area to gross occupied surface area is comparatively low. Stationary solar energy collectors collect solar energy from a more limited range of acceptance angles than do tracking collectors, but do have the advantage of providing a high ratio of the net collector area to the gross surface area occupied by the entire collector apparatus since little adverse shading occurs. Therefore, stationary concentrating collectors make good use of available support space.
Whether of the tracking or stationary type, each of the concentrating solar energy collectors disclosed in the patents and papers noted above is characterized by certain inefficiencies in its design. Specifically, in some collectors, the absorber is mounted at a location spaced from and therefore out of contact wih the reflector. Examples of this configuration are shown in Lightfoot, Parker, Hurkett and Holt patents and in the CPC collector shown in the Patton paper. Therefore, at least some solar energy may be reflected by the reflector through the region between the absorber and the reflector, referred to as the gap or COR, to, in turn, be reflected back outwardly from the collector. A substantial amount of energy received by the reflector is not received by the absorber and cannot be used when such a cap or COR exists.
In other designs, the absorber is mounted directly on or is otherwise connected to the reflector. Examples of such an arrangement are shown in the L'Esperance et al., Barrio de Mendoza and Eshelman patents and in the CPC collector shown at the Cole et al. paper. Therefore, at least some of the energy that would have been reflected between the absorber and reflector in designs in which these components are separated is intercepted by the absorber or the members which connect the absorber to the reflector. However, heat generated in the absorber by the collected energy may be conducted back into the reflector through its direct or indirect contact with the absorber instead of being conducted away by a heat transferring medium for subsequent use. Therefore, a substantial amount of energy received by the collector is not utilized but is rather conducted inefficiently into the apparatus. Furthermore, rapid degradation of the surface of the reflector may be caused by this contact because the mounting member or absorber and the reflector are usually made from dissimilar materials and because of the high temperatures at which the absorber operates.